Tightening would involved lengthening the deck slightly. There should be(on most treadmills i've seen) two screws of some kind at the end without the motor.
Generally they are used to adjust the belts direction as some people naturally walk with a slight pull to one side because they tread heavier one side than the other which pulls the mat one way or the other. You are supposed to use the screws quarter of a turn at so the mat tends to pull the opposite direction to the way you pull the mat.
It doesn that by changing the length of the end runner, so in your case if the whole mat is slipping you probably want to adjust both screws a quarter of a turn at a time. Hopefully you can see which direction its moving, away from the motor will be tighter. Then test it for a while between each change. As long as the mat isn't pushing left or right as you use it now make sure whatever adjustments you make are equal on both sides.
There can be other reasons for slipping though, underside of the mat being too lubricated, worn down, probably other issues.
Personally I messed around with a couple cheap treadmills, then I went all in, got a great deal on a higher end treadmill, was £1400 but was about to be replaced so got it for £750ish, ridiculously better quality than the cheap one I had, no problem running full speed on it, feel's so much safer and better, sturdier aswell and much more comftable on my stupidly bad knee's.
